Many mirrors and other flat or curved implements are manufactured in sandwich form in order to provide an overall structure that is strong, rigid, and light. A typical sandwich structure is comprised of a support core surrounded by two face plates: a front plate and a back plate.
An example of an application for such sandwich structures are Large Deployable Reflectors (LDR). LDR reflectors are typically comprised of a grid of Precision Segmented Reflectors (PSR). The construction of the sandwich cores used in such an application is of paramount importance because the required accuracies (as low as 3 microns in some cases). Inaccuracies may be introduced by manufacturing tolerances where the core is rigid and must be machined to the curvature of the face plates, by uneven flexing characteristics where the core is flexible, and by expansion or contractions due to the harsh temperature environments such as LDR's may encounter.
One known core design has a hexagonal honeycomb configuration and is known as hex core (see FIG. 8). The problem with hex core is that it is rigid and cannot naturally flex in order to conform to the curvature of the face plates. Instead, both sides of the hex core must be machined to curvatures that mate precisely with the face plates. The machining process is time consuming and expensive. Moreover, inaccuracies are inevitable because of ordinary machining tolerances. When bonded together, any inaccuracy between the curvature of the machined hex core and the face plates lead to complex errors such as astigmatism.
Flexible cores that, unlike hex core, can be bent to a given curvature are known (see FIG. 9). For example, a flexible core is manufactured under the trade name FLEX-CORE by Hexcel Corporation. The problem with the known flexible cores is that their geometries are such that the cores can only bend uniformly in one direction. The known flexible cores are therefore unsuitable for supporting complex curved surfaces because the stresses created in the flexed core are transferred to the curved surface.
Thermal stability is also an important concern because curved sandwich structures are typically subject to wide temperature variations. For example, a lightweight mirror or antenna located in outer space may experience temperatures as low as 100 Kelvin and as high as 300 Kelvin. The geometry of the core and the overall design of the sandwich structure is very important under such extreme temperature variations.
First, the sandwich structure should be of uniform thickness so that thermal effects are uniform throughout. Although hex core can be machined on both sides to provide a curved core of uniform thickness, it can only be so machined at great effort and expense.
Second, the core, whether flat or curved, should be self-venting so that no air is trapped in the core between the face plates. Trapped air can lead to a debond between the facesheets and the core.
Third, where precise figure control is required, it is very important to minimize temperature-based stresses by ensuring that all three of the sandwich structure components have very low and equal coefficients of thermal expansion (CTE). The known flexible cores are often unsuitable for extreme temperature environments because they are only available in a limited number of materials. For example, FLEX-CORE is not available in graphite composite materials, materials that are commonly used to fabricate the faces plates because of their rigidity, lightness, and extremely low CTE characteristic.